This invention relates to a means for determining corrosive conditions and a process for its use. The invention particularly relates to real-time detection of the initiation of localized corrosion, thereby enabling commencement of appropriate anti-corrosion counter measures to prevent or minimize equipment damage.
Heat transfer equipment (heat exchangers) in contact with an electrolyte is subject to corrosion. Corrosion is the primary cause of system failures resulting in high maintenance labor and lost production. To inhibit corrosion, various types of chemical inhibitors are applied along with other supporting chemical treatments in an effort to control the nature and rate of corrosion. It has long been desired to be able to optimize inhibitor concentrations based on real-time program performance. However, such attempts have met with only minimal success.
The two categories of inhibitors common to cooling water treatment applications are cathodic and anodic inhibitors. Water treatment programs may incorporate one or more corrosion inhibitors based on the method(s) of corrosion inhibition. Also, specialty polymers are commonly applied with the selected corrosion inhibitor to inhibit uncontrolled precipitation of the corrosion inhibitor. Therefore, in order to select the appropriate inhibitor(s) and optimize their concentration in the electrolytic solution, e.g. cooling water, the nature of corrosion needs to be accurately determined.
It is desirable to inhibit localized corrosion upon detection, e.g. to minimize the severity of the pitting of carbon steel and/or to minimize stress corrosion cracking on stainless steel induced, for example, by the presence of chlorides, as well as to reduce the insulating effects caused by the accumulation of corrosion byproducts. This is particularly critical in high temperature heat exchangers, where extending the period the metal is exposed to localized corrosion can induce premature failure of the heat transfer equipment.
Many systems experience periodic upsets that can dramatically increase the onset and rate of corrosion. These upsets can result from numerous process deviations, e.g. increased concentrations of corrosion inducing ions in the incoming water (such as chlorides and sulfates) or process leaks that contaminate the water. During upset conditions, localized corrosion is usually the dominant method of corrosion. If the concentration of inhibitor(s) in the cooling water is not adjusted to compensate for the upset, severe damage to the heat transfer surfaces, as well as supporting equipment results.
Prior art processes teach of storing data, e.g. electrochemical noise data (ECN), and using said data to calculate a slope of the amplitude to determine the nature of the corrosion. This methodology necessitates extensive collection of data over time. During the period of collecting these values and determining the slope generated from the data obtained from a Fourier Transform, propagation of pitting, metal dissolution, and accumulation of corrosion byproducts continues unabated, thereby compromising the equipment integrity. Since the prior art methodology requires as much as hours of inputs and interpretation thereof, localized corrosion on heat transfer equipment will continue to occur.
Corrosion inhibitors are currently controlled by various devices in an attempt to maintain a predetermined concentration. The desired inhibitor concentration is determined based on guidelines established via test data, field experience, and historical data obtained by site specific monitoring. This methodology fails to appreciate the criticality of real-time detection to minimize the deleterious effects of corrosive conditions.
Thus, in order to quickly suppress corrosion, the nature of corrosion must be identified so that appropriate selection and levels of inhibitor(s) are administered. For example, because of the severity of localized corrosion, identification of pitting corrosion at the moment of pit initiation is highly desirable. If the pitting can be quickly detected and identified during the initial stage and inhibited before propagation of the pitting, minimal loss of metal and accumulation of corrosion byproducts occurs. System integrity is managed, resulting in improved operational performance, increased reliability and equipment life.
U.S. Pat. No. 4,575,678 describes an apparatus having two metal parts corroding in an electrolyte and separated by an insulator. The low frequency voltage and current between the electrodes is observed. This low voltage and current is a low frequency noise signal. Amplitude values of the signals are measured and computed yielding data indicating the corrosion rate and the nature of corrosion. This method of corrosion measuring is commonly referred to as electrochemical noise (ECN).
U.S. Pat. No. 5,888,374 (US Government has rights pursuant to contract with Argonne National Laboratory) describes a means of monitoring localized pitting corrosion. The patent describes storing values from Electro-Chemical Noise sensors and processing the stored values utilizing Fourier Transform. A slope of the power spectral density data relative to frequency is calculated. The data is trended over time (hours) to determine the subtle deviations in the slope.
The prior art fails to recognize the need for real-time detection and evaluation of corrosion conditions and the need to integrate such data with corrosion inhibiting measures.
This invention describes a novel means of determining the nature (type) of corrosion in real-time. The real-time analysis of upset conditions has now been recognized as a valuable tool in both the detection and forecasting of localized corrosion. This is especially useful when replicating the operational and environmental conditions between metals and electrolyte in heat exchangers. By identifying localized corrosion at the moment of pit initiation, real-time selection, and effective concentration(s) of appropriate inhibitor(s) can be delivered to an electrolytic solution before propagation of the localized corrosion.
Prior art processes have historically relied on a methodology requiring the storing of electrochemical noise data, and using said data to calculate a slope to determine the nature of the corrosion. The delay in identifying the onset and type of corrosion inherent in this methodology often results in severe, and sometimes catastrophic, damage to associated equipment. Furthermore, heat transfer properties are often compromised due to the insulating effects caused by the accumulation of corrosion byproducts.
The instant method provides real-time identification of localized corrosion during the initial stage of formation. Integrating this invention with corrosion inhibitor feed can effectively inhibit the corrosion before propagation occurs, thereby maintaining system integrity. The process embodies a method of operation wherein the electrochemical noise (ECN) and linear polarization (LPR) values are processed to compare how the corrosion signals correlate. Divergence of the corrosion signals indicates the formation of localized corrosion while continuity in signal pattern indicates generalized corrosion.
Accordingly, it is an objective of the instant invention to teach a device and a process for its use which can identify corrosion, particularly localized corrosion, in real-time during the initial stage of pit formation.
It is an additional objective of the instant invention to integrate the corrosion identifying process with a process for corrosion inhibition which can quickly suppress the corrosion cell(s) by making real-time adjustments to the inhibitor program.
It is yet an additional objective of the instant invention to teach a process for monitoring corrosion values for both electrochemical noise (ECN) and linear polarization (LP) under heat transfer.
It is still an additional objective of the instant invention to teach a method of operation wherein the ECN and LP values are processed to compare how the corrosion signals correlate.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.